In general, a conventional image sensor is mainly used in a CIF(common intermediate format) scale composed of 352 pixels×288 pixels, or a VGA (video graphic array) scale composed of 640 pixels×480 pixels. Recently, the image sensor for high resolution greater than one Mega-pixel is mainly traded in an image sensor market. As resolution of the image sensor is greater than one Mega- pixel, new problems not shown in the conventional CIF scale and VGA scale have issued.
Generally, noises occurred in the image sensor can be mainly classified into a vertical noise and a horizontal noise. The vertical noise caused by a fixed pattern noise (FPN) can be mainly removed by a correlated double sampling (CDS) technique. On the other hand, the horizontal noise occurred in the high resolution image sensor is caused by parasitic capacitances between a gate and a drain of each load transistor. Hence, it is hard to remove the horizontal noise.
FIG. 1 is a picture showing a horizontal noise occurred in a 1.3 mega-pixel product on developing.
As shown in FIG. 1, the horizontal noise appears in a region of a bright object on screen according to a horizontal direction. The horizontal noise occurs because pixels located in a row including the predetermined pixel corresponding to the bright object are smaller than neighboring pixels located in upper or lower rows of the predetermined pixel corresponding to the bright object.
The horizontal noise occurs in the image sensor for high resolution more than one Mega-pixel because load transistors operated by a bias current increase more than conventional load transistors to thereby increase total parasitic capacitances between gates and drains of load transistors.
This structural problem can be improved by increasing the bias current, however the current of each column analog bus also increases accordingly to thereby decrease a dynamic range of the pixels and deteriorate an optic characteristics.